1. Field of the Invention
The present invention relates to video synchronization signal generators, and in particular, to computer controllable video synchronization signal generators.
2. Description of the Related Art
Modern video systems typically require the use of numerous peripheral video subsystems which must all be synchronized with one another. Further, most sophisticated video systems, such as video signal processors or work stations, require the use of a computer to control the various video peripherals or to perform some form of data processing. Therefore, a video synchronization ("sync") signal source is required for supplying each of the peripheral video subsystems with a common video sync signal. Further required is a means by which the computer can interface with the various video peripherals so as to supply them with appropriate control signals and to receive from the video peripherals any data it requires.
Typically, a stand-alone video sync signal source, several of which are known in the art, is used to supply a common video sync signal source, such as a video black burst signal. Its output signal, i.e., the video sync signal, is coupled to each video peripheral. A drawback to this, however, is that several video sync signal sources are required if video sync signals based upon several different video standards (e.g., NTSC, PAL, SECAM) are desired. Furthermore, even if several such video sync signal sources are available, selecting the desired video sync signal from among these sources requires the manual disconnection and reconnection of the various video peripherals to the desired video sync signal source. Alternatively, the computer can be used to control a video signal switch to select from among the various video sync signal sources. However, this requires the use of yet another computer interface.
The interface between the computer and the various video peripherals is typically accomplished by way of a standard interface protocol, such as RS422. With a serial RS-422 interface, each peripheral can communicate with the computer to indicate when the peripheral is in need of servicing, e.g., when it needs instructions or has data available for processing by the computer. However, transfer of instructions or data between the computer and peripherals must take place only during specific time periods within each video frame.
This requires the use of some form of video frame pulse signal, synchronized to the common video sync signal, which can be used for determining the proper timing for transfer of instructions or data. Based upon this video frame pulse signal, an interrupt request signal must then be generated by each peripheral to send to the computer. Thus, each video peripheral must be provided with such a video frame pulse signal, or alternatively, have some decoding circuitry for decoding an appropriate frame pulse signal based upon the common video sync signal.
If a video frame pulse signal must be distributed along with the video sync signal, the aforementioned dilemma associated with multiple peripherals becomes worse, i.e, having to manually disconnect and reconnect multiple peripherals or provide a computer controlled video signal switch when several video sync signals are used (e.g., NTSC, PAL, SECAM). Further, such decoding circuitry can be rather complex if video sync signals based upon several video standards are to be used within the video system.
Therefore, it would be desirable to have a self-contained unit in which a computer controllable switching means can programmably select a common video sync signal from among multiple video sync signals which are based upon different video standards. It would be further desirable to have as part of such a unit a common video sync signal source from which a common video frame pulse signal can be derived and used by the computer for determining the proper timing for transferring of instructions or data.